I know none of the suggestions I have read here will work with my students. Adults already feel like children (not in a good way) trying to speak a foreign language and treating them like kids can only make that worse.

Rather than punishing them for speaking L1 my thing is to praise them and encourage them as much as possible to speak English, explain things to each other in English, etc.

I am not easily exasperated so I will sometimes just say "English, please" in L1 as an ironic joke.